[Blocked] Add empty value attribute

CHANGELOG.md

@@ -33,6 +33,8 @@ release.
 
 - ⚠️ **IMPORTANT**: Extending the set of standard attribute value types is no longer a breaking change.
   ([#4614](https://github.com/open-telemetry/opentelemetry-specification/pull/4614))
+- Add empty value attribute.
+  ([#4595](https://github.com/open-telemetry/opentelemetry-specification/pull/4595))
 
 ### Supplementary Guidelines
 

specification/common/README.md

@@ -8,7 +8,7 @@ path_base_for_github_subdir:
 
 # Common specification concepts
 
-**Status**: [Stable](../document-status.md)
+**Status**: [Stable](../document-status.md), except where otherwise specified
 
 <details>
 <summary>Table of Contents</summary>
@@ -38,15 +38,13 @@ An `Attribute` is a key-value pair, which MUST have the following properties:
   - A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
   - An array of primitive type values. The array MUST be homogeneous,
     i.e., it MUST NOT contain values of different types.
+  - **Status**: [Development](../document-status.md) - An empty value (e.g. `null`).
 
 For protocols that do not natively support non-string values, non-string values SHOULD be represented as JSON-encoded strings.  For example, the expression `int64(100)` will be encoded as `100`, `float64(1.5)` will be encoded as `1.5`, and an empty array of any type will be encoded as `[]`.
 
-Attribute values expressing a numerical value of zero, an empty string, or an
-empty array are considered meaningful and MUST be stored and passed on to
-processors / exporters.
-
-Attribute values of `null` are not valid and attempting to set a `null` value is
-undefined behavior.
+Attribute values expressing an empty value, a numerical value of zero,
+an empty string, or an empty array are considered meaningful and MUST be stored
+and passed on to processors / exporters.
 
 `null` values SHOULD NOT be allowed in arrays. However, if it is impossible to
 make sure that no `null` values are accepted

specification/common/attribute-type-mapping.md

@@ -13,6 +13,7 @@ linkTitle: Mapping to AnyValue
 
 - [Converting to AnyValue](#converting-to-anyvalue)
   * [Primitive Values](#primitive-values)
+    + [Empty Value](#empty-value)
     + [Integer Values](#integer-values)
     + [Enumerations](#enumerations)
     + [Floating Point Values](#floating-point-values)
@@ -54,6 +55,12 @@ follow the rules described below.
 
 ### Primitive Values
 
+#### Empty Value
+
+Empty values MUST be converted to AnyValue with no
+[value](https://github.com/open-telemetry/opentelemetry-proto/blob/38b5b9b6e5257c6500a843f7fdacf89dd95833e8/opentelemetry/proto/common/v1/common.proto#L28-L30)
+field being set.
+
 #### Integer Values
 
 Integer values which are within the range of 64 bit signed numbers